The present invention relates to devices for analyzing and sampling combustion gases and, more particularly, to a new and useful device for sealing and isolating aspirated gas sample flows from pressurized supply air flow.
The control of combustion systems is often dependent upon the accurate monitoring and analyzing of process gases such as combustion flue gases. Known sampling systems for measuring concentrations of oxygen carbon monoxide and other combustibles include forced or pressurized type systems and aspirated or vacuum type systems. In the pressurized systems, small amounts of leakage between the system and the environment are not critical as leakage is vented to atmosphere due to the higher sampling system pressure. In aspirated systems, however, leakage between the sampling system and environment or supply air and sampled gas channels causes the higher pressure air to flow into the sampled gas. This inflow of air increases the oxygen content of the sample and thereby results in an erroneous measurement.
The aspirators in one known arrangement, such as is disclosed in U.S. Pat. No. 4,134,289, as well as the various inlet and outlet lines associated therewith are all enclosed in a heated block to maintain the temperature well above the dew point of any sampled gases or supply air in order to prevent condensation. In the case of such high temperature analyzing, in which temperatures may exceed 450.degree. F. (232.2.degree. C.), metal, ceramic and asbestos materials are employed to form analyzer block parts. The contents of U.S. Pat. No. 4,134,289 are incorporated herein by reference.
The formation of fluid flow passages in the block, particularly in respect of serpentine, sinusoidally extending channels used to assure sufficient residence time to heat the fluid flows, can be facilitated by machining or otherwise forming surface grooves in a sealing face of a flat plate, such as a manifold flange, and joining the flange together with a seal plate or the like so that the adjacent surfaces establish distinct, separate flow paths. To assure adequate sealing between mating parts, however, such parts must be machined to close tolerances, be formed with microsmooth finishes and precision flatness, and then joined under high clamping loads which are uniformly distributed over the mated surfaces. In operation, small leakages may nevertheless occur due to deviations from design specifications, imperfections in the sealing surfaces such as scratches or burrs, due to dirt or particles which will sometimes get between the sealing surfaces, or even due to thermal distortion.